1) Field of the Invention
The present invention relates to stringers and, more particularly, to a collapsible mandrel having reinforcing strips therein that may be employed to co-cure stringers, as well as an associated fabrication method.
2) Description of Related Art
Aircraft utilize a variety of elements for reinforcement due to the application of bending, torsional, shear, and direct loading on the aircraft. One of the basic reinforcing structures used within aircraft include stringers (also termed stiffeners and longerons), which typically run spanwise within the wing. Stringers may be fabricated into various cross sections, such as I-beams, Z, and “hat” sections. Stringers are typically a lightweight composite formed into a tape or fabric, which includes using a plurality of fibers embedded in a matrix of resin or adhesive to support and bond the fibers together (i.e., pre-impregnated or “prepreg”). A lay-up method is typically used to form the stringers, wherein the skin, typically also formed as a prepreg, is applied to a stringer using sufficient head pressure. Autoclaving or a similar method is then used to cure the stringer and the aircraft skin together.
There are a variety of mandrels used to co-cure stringers. For example, there are hollow mandrels that, when vented to an autoclave, press from the inside out. However, the hollow mandrels cannot support the tape head rolling over the stringers. Another type of mandrel includes hard rubber mandrels placed between the skin and stringer, but this mandrel has proved to be difficult to remove from long stringers following curing. There are also expendable mandrels that may be dissolved after curing, but this mandrel requires a messy and impractical dissolution process. Finally, there is a re-usable elastomeric mandrel system (REMS), which is essentially a rubber shaped mandrel full of ceramic beads. In spite of this design, the REMS is impractical to use with longer stringer lengths.
Further, U.S. Pat. No. 5,368,807 to Lindsay discloses a method for vacuum bag molding fiber reinforced resin matrix composites without requiring the use of a mandrel. The method includes laying prepreg stiffener hat sections on the inside portion of a rigid portion of a vacuum bag. An elastomeric tube is inserted within each of the stiffener sections, and may be inflated to conform to the shape of the stiffener. A sealing wedge is placed on the end of each of the stiffeners to hold and seal the elastomeric tube to the rigid portion of the vacuum bag. Following curing, the wedge is removed, and a vacuum drawn on the elastomeric tube is used to collapse the tube for removal from the vacuum bag.
Lindsay shows the stringers or longerons arranged linearly, but does not permit arranging and co-curing the stringers in a variety of other contours, as would be desirable. In addition, the rigid vacuum bag only supports the stringer during autoclaving but does not support the stringer during laying up of the aircraft skin. Thus, the aircraft skin in Lindsay is not capable of being applied over a recessed area of a “hat-shaped” stringer such that the elastomeric tube does not have to withstand the head pressure of tape laying aircraft skin to the stringer, where the head pressure may also cause collapse of the aircraft skin within the recess or of the stringer itself.
It would therefore be advantageous to provide a mandrel used in co-curing stringers that may withstand the pressure applied during lay-up. Also, it would be advantageous to provide a stringer and mandrel that may be formed into various shapes prior to co-curing. Finally, it would be advantageous to provide a mandrel that may be easily removed following co-curing.